In an optical disc recording and reproducing apparatus, in which information is recorded with respect to an optical disc, or information is reproduced from the optical disc having the information recorded therein, it is needed to control the position of each of many mechanical parts. Among such many mechanical parts, especially with respect to the position control for an objective lens of the optical head (optical pick-up), a high-speed and high-precision control needs to be performed.
Taking up the focus servo for an objective lens as an example, whereas the optical disc makes a plane vibration of several hundreds of μm, the objective lens has a permissible error of 100 nm or less. Therefore, it is necessary to decrease the steady-state deviation of the objective lens as greatly as possible. In addition, since the range within which a focus control error signal for the objective lens is generated is only within several μm, even when the steady-state deviation is very small, if the overshoot is large when starting the control of the focus servo, the focus servo goes beyond the pull-in range. As a result, it becomes impossible to normally start the control of the focus servo.
In the case of a tracking servo for the objective lens, with a decrease in the track pitch of the optical disc, the problem exists that the pull-in operation of the tracking servo cannot be stabilized due to a very small overshoot at the time of starting the servo.
First, with reference to FIG. 11, a focus servo which is used in a conventional optical disc recording and reproducing apparatus will be explained. By discrimination means 37, it is discriminated whether a focus drive changeover switch 34 is in an “ON” state. If “NO”, i.e. the focus drive changeover switch 34 is in an “OFF” state and so the input side of a drive circuit 35 is in a grounded state, the focus drive turned “OFF”. A focus error signal which is obtained by calculating the optical-detection output signal from a multi-divisional photo-detector of the optical head is supplied to an AGC (Automatic Gain Control) circuit 31 and, in correspondence with the level of a Pull-In signal, has its level automatically controlled. Then, the signal is supplied to a phase compensation circuit 33 and its phase is compensated.
When “YES” discrimination is made by the discrimination means 37, namely when the focus drive changeover switch 34 is turned “ON”, the focus search signal and the focus error signal from the phase compensation circuit 33 which has its level automatically controlled and has its phase-compensated are changed over between them, under the control of the focus search “ON” signal and the focus “ON” signal, by the focus search drive changeover switch 34, and this signal is supplied to the drive circuit 35.
For a while after the focus drive changeover switch 34 is turned “ON” from its “OFF” state, it is discriminated by discrimination means 38, after the absolute value of the focus error exceeds a set value, whether the absolute value of the focus error is smaller than a predetermined threshold value εfe and, in addition, the pull-in signal is greater than a threshold value PIon. When “NO” discrimination is made by the discrimination means 38, the focus drive changeover switch 34 is changed over to the focus search “ON” side by the focus search “ON” signal, and the focus search signal is supplied to the focus drive circuit 35 via the changeover switch 34. And, by the focus drive signal from that drive circuit 35, a focus actuator 36 is driven.
When, thereafter, “YES” discrimination is made by the discrimination means 38, the focus is turned “ON” by the focus “ON” signal, and then the focus drive changeover switch 34 is changed over to the phase compensation circuit 33 side, whereby the focus error signal whose level is automatically controlled and whose phase is compensated is supplied to the drive circuit 35 via the focus drive changeover switch 34. And, by the focus drive signal from that drive circuit 35, the focus actuator 36 is driven, whereby the servo loop is closed.
Here, attention is paid to the focus drive signal at the moment when the focus drive changeover switch 34 has been turned “ON”, namely the focus error signal that has been outputted from the phase compensation circuit 33. The characteristics of the gain G and phase (θ) of the phase compensation circuit 33 of FIG. 11 with respect to the frequency are illustrated in FIGS. 12A and 12B, and hereafter an explanation will be given of these FIGS. 12A and 12B. The characteristic of the gain G of the phase compensation circuit 33 with respect to the frequency is set such that when the frequency f increases the gain G may become higher than when the frequency f is low. Namely, the frequency in the high range in the focus error signal is stressed. On the other hand, the characteristic of the phase θ of the phase compensation circuit 33 with respect to the frequency is set such that, with a frequency f=fc as the border, in the higher range the phase may be advanced and in the lower range the phase may be lagged.
Accordingly, the polarity of the focus drive signal immediately after the turning-“ON” of the focus servo, namely the focus error signal from the phase compensation circuit 33 does not always become a servo signal that makes the relative speed between the objective lens and the optical disc low.
Next, with reference to FIG. 13, an explanation will be given of the focus error signal and the phase-compensated focus error signal. FIGS. 13A and 13C are waveform views respectively illustrating changes of the focus error signal FE1 with respect to time t. FIGS. 13B and 13D are waveform views respectively illustrating changes of the phase-compensated focus error signal (focus drive signal) FE2 with respect to time t.
As illustrated in FIGS. 13A and 13B, in a case where the phase of the focus drive signal FE2 illustrated in FIG. 13B is advanced relative to the focus error signal FE1 illustrated in FIG. 13A, the moment the focus servo is closed (the servo is turned “ON”), the objective lens is driven in the direction of making the relative speed between the objective lens and the optical disc low. As a result of this, the excellent pull-in operation of the focus servo for the objective lens is performed.
In contrast to this, as illustrated in FIGS. 13C and 13D, in a case where the phase of the focus drive signal FE2 illustrated in FIG. 13D is lagged relative to the focus error signal FE1 illustrated in FIG. 13C, at the instant the focus servo loop is closed (the servo is turned “ON”), the objective lens is driven in the direction of making the relative speed between the objective lens and the optical disc high. Therefore, an overshoot of the focus servo is generated. The τ in FIG. 13D indicates the period during which the relative speed is increased. Since the pull-in operation range of the focus servo at the time of driving the optical disc to rotate is as very narrow as several μm or less, it happens that the pull-in operation fails due to a very small overshoot at the time of the turning-“ON” of the servo.
Next, with reference to FIG. 14, an explanation will be given of a tracking servo which is used in the conventional optical disc recording and reproducing apparatus. By discrimination means 47, it is determined whether a tracking drive changeover switch 44 is “ON” or not. If “NO”, namely the tracking drive changeover switch 44 is “OFF” and the input side of a drive circuit 45 is kept grounded, the tracking drive becomes turned “OFF”.
A tracking error signal which is obtained by calculating the optical-detection outputs from the multi-divisional photo-detector of the optical head is supplied to an AGC (Automatic Gain Control) circuit 41, and, in correspondence with the level of the Pull-In signal, the level of that tracking error signal is automatically controlled and then this signal is supplied to a phase compensation circuit 43 so that its phase is compensated.
When “YES” discrimination is made by the discrimination means 47 and the tracking drive changeover switch 44 is turned “ON”, a tracking jump signal and the tracking error signal from the phase compensation circuit 43 which has its level automatically controlled and has its phase-compensated are changed over between them by the tracking drive changeover switch 44, under the control of a tracking jump “ON” signal and a tracking “ON” signal. And, this changed-over signal is supplied to the drive circuit 45. For a while after the tracking drive changeover switch 44 is changed over from “OFF” to “ON” state, it is discriminated, by a monitor for a tracking gate signal, whether the tracking “ON” gate signal has a high level. When the judgment of the discrimination means 49 is “YES”, the tracking servo goes “ON” while when “NO” the tracking servo goes “OFF”.
The tracking “ON” gate signal has a high level when the following occurs. Namely, it has a high level when the absolute value of the tracking error signal of FIG. 9A as later described is smaller than a set value TEzc and when the absolute value of the peak-held tracking error signal after passing through a high boost filter shown in the FIG. 9B is smaller than a set value TEhb as described later on.
After the tracking servo is turned “ON”, it is discriminated, by discrimination means 48, whether the tracking servo is “ON” and whether now is the tracking jump timing or not. When “YES”, namely the time is the tracking jump timing, the tracking jump is turned “ON”. When, by the tracking jump “ON” signal, the tracking drive changeover switch 44 is changed over to the tracking jump signal side, this tracking jump signal is supplied to a tracking drive circuit 45 via the tracking drive changeover switch 44. As a result of this, by the tracking drive signal from the drive circuit 45, a tracking actuator 46 is driven.
Incidentally, at the time, as well, of starting this tracking servo operation, for the same reason as in the case of the focus servo operation, it sometimes happens that an overshoot occurs in the vicinity of an adjacent track. This becomes an obstacle to stably starting the tracking servo operation. This tendency is becoming more and more prominent as the track pitch decreases with the increase in the density of the information on the optical disc.
In view of the above-described points in problem, the present invention is intended to propose a position control apparatus and position control method, a recording apparatus and method for an optical disc, and a reproducing apparatus and method for an optical disc, in each of which, even when the range within which a mechanical part is pulled in for position control is small, the pull-in operation can quickly be stabilized.